Density Functional Theory Study Of Several Catalytic Reactions Over Zeolites

Zeolites are inorganic crystalline materials with regular channels and cavities to form constrained environments for molecules,which can be used to recognize molecules and control the transition states by their unique shape or size selectivity.Zeolites are widly used in practical applications of catalysis,ion exchange,adsorption and separation,etc.Traditional zeolites are based on aluminosilicates,in which the silicon and aluminium atoms are linked through oxygen atoms to form the three-dimensional frameworks.Zeolites as the solid acid catalysts play an important role in petroleum and petrochemical industries.The reaction enthalpies and entropies of adsorption and catalytic reactions are related to the confinement effects of pore size and shape of zeolite channels and cavities.Only the facile guest molecules can pass through out zeolite channels,and the catalytic process is influenced by the reactants,products,transition state and molecular traffic control.In recent years,with the rapid development of computer technology,the quantum chemical calculations as the most effective research method have been used in the fields of chemistry,physics,biology,pharmacy and engineering.In particularly,there has been much progress on the quantum chemical calculations of zeolite materials.The quantum chemical calculations can help to understand the phenomenon of adsorption and diffusion,and provide further insight into the mechanism of catalysis reactions.Therefore,quantum chemical calculations have become an indispensable research tool for zeolite research.In this thesis,we employ the density functional theory(DFT)calculations to investigate several catalytic reactions over zeolites,including:(1)investigation of the confinement effect of zeolite cavities on the methanol-to-olefin(MTO)conversion;(2)prediction of the shape selectivity of hypothetical zeolite cavities for MTO conversion;(3)study of the catalytic mechanism of the side-chain alkylation from toluene with methanol over zeolite X;(4)illustration of the catalytic mechanism of the hydroxyl free radicals in accelerating the zeolite crystallization.These calculation results will be helpful to understand the catalytic process at molecular level and further to design effective catalysts.The main results are summarized as follows:1.The confinement effect of zeolite cavities on the MTO conversion is investigated through DFT calculations.According to the side-chain mechanism,we select several hydrocarbon pool(HP)intermediates that may exist during the MTO conversion process and optimize their structures within the cluster models of zeolite cavities cha,lev,and lta,respectively.The transition states during methylation,deprotonation,methyl shift,and olefin-production are also located within these cavities.According to our results,all of the HP intermediates are stabilized in zeolite cavities,especially in cha and lta.The cha cavity displays the lowest intrinsic free-energy barriers for all of the methylation and olefin-production steps,indicating its high MTO catalytic activity.We find that the differences in reaction barriers and reaction energies are highly related to the different confinement effects of zeolite cavities.In comparison with lev and lta,the cha cavity matches the dimensions of HP species very well,so it is able to provide the most suitable confinement to HP species.Our study will provide further understanding of the side-chain mechanism,which is important for finding new catalysts for MTO conversion.2.On the basis of the confinement effect of zeolite cavities,we further study the catalytic performance of the abc-6 zeolites cavities including the known and hypothetical structures on the MTO conversion.There are 56 types of zeolite cavities in the abc-6 zeolite family.Firstly,we calculate the volume of all the abc-6 zeolite cavities,and selecte the candidate cavities,that have suitable volume,shape and size like cha cavity.And then we exclude the cavities,that are exclusively built by six-rings.Finally,7 types of zeolite cavities are investigated into the catalytic performmance of MTO conversion by using the DFT calculations.The calculation results show that,the aft and avl cavities displayed high catalytic activity in the methylation.In addition,the avl cavity has the lowest intrinsic reaction barriers of the olefin-production steps,indicating it would be the best candidate for the MTO conversion.Our investigation can scanning the catalytic performance of the abc-6 zeolite cavites,which is important for predicting new zeolite catalysts for MTO conversion.3.The catalytic mechanism of the side-chain alkylation from toluene with methanol over alkali cations exchanged zeolites X is investigated through DFT calculations.Two reaction pathways of alkylation from toluene with methanol based on the base and acid sites of zeolite X are performed by a six membered-ring(6MR)cluster.The calculation results show that the base site can activate the methanol to formaldehyde and stabilize the side chain of toluene.Meanwhile,the acid site can facilitate the formation of the styrene and ethylbenzene with lower barriers for dehydration and hydrogenation reactions than that on base sites.According to the basic reaction pathway,zeolite X with Cs+ cation possesses the lower reaction barrier for the rate-determining step of side-chain alkylation than the other alkali cations exchanged zeolites X.The calculations also show that the formation of the styrene and ethylbenzene could be facilitated by the acid site,which deduces the reaction barriers of dehydration and hydrogenation reactions in side-chain alkylation process.Our theoretical study reveals that the side-chain alkylation from toluene with methanol is manipulated in a cooperative action of the acid and base sites of the alkali cations exchanged zeolites X.All of these results agree well with the experimental observations.This theoretical investigation will be helpful to understand the catalytic process at molecular level and further to design effective catalysts.4.The interaction of the hydroxyl free radicals and the hydroxide ions with the silicon hydroxyls is studied by the DFT calculations.Two reactions of dissociation of Si-O-Si bond and condensation of the silicon hydroxyls are investigated by 2T cluster models.We optimize the reactants and products at UB3LYP/6-31g++(d,p)level and the single-point energy calculations are performed at M0-62X/6-31g++(d,p)level to calculate the free energy barrier and free energy of reactions(in kcal/mol).The calculation results show that the reaction mechanism of the breaking and remaking of the Si-O-Si bonds catalyzed by the hydroxyl free radicals is more active than hydroxide ions.We also calculate the reaction mechanism of the hydroxyl free radicals reacted with water to form Si-O· radicals.The calculation results show that the formed Si-O· radicals are more active than hydroxyl free radicals,and the formed Si-O · radicals are easily consumed to generate hydroxyl free radicals,which is consistent with the observation that no Si-O· radicals are detected in all above experiments.